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. 2019 Aug 14;141(32):12804-12814.
doi: 10.1021/jacs.9b05740. Epub 2019 Aug 2.

Assembling a Natural Small Molecule into a Supramolecular Network with High Structural Order and Dynamic Functions

Qi Zhang  1 , Yuan-Xin Deng  1 , Hong-Xi Luo  2 , Chen-Yu Shi  1 , Geoffrey M Geise  2 , Ben L Feringa  1   3 , He Tian  1 , Da-Hui Qu  1
Affiliations

Assembling a Natural Small Molecule into a Supramolecular Network with High Structural Order and Dynamic Functions

Qi Zhang et al. J Am Chem Soc. .

Abstract

Programming the hierarchical self-assembly of small molecules has been a fundamental topic of great significance in biological systems and artificial supramolecular systems. Precise and highly programmed self-assembly can produce supramolecular architectures with distinct structural features. However, it still remains a challenge how to precisely control the self-assembly pathway in a desirable way by introducing abundant structural information into a limited molecular backbone. Here we disclose a strategy that directs the hierarchical self-assembly of sodium thioctate, a small molecule of biological origin, into a highly ordered supramolecular layered network. By combining the unique dynamic covalent ring-opening-polymerization of sodium thioctate and an evaporation-induced interfacial confinement effect, we precisely direct the dynamic supramolecular self-assembly of this simple small molecule in a scheduled hierarchical pathway, resulting in a layered structure with long-range order at both macroscopic and molecular scales, which is revealed by small-angle and wide-angle X-ray scattering technologies. The resulting supramolecular layers are found to be able to bind water molecules as structural water, which works as an interlayer lubricant to modulate the material properties, such as mechanical performance, self-healing capability, and actuating function. Analogous to many reversibly self-assembled biological systems, the highly dynamic polymeric network can be degraded into monomers and reformed by a water-mediated route, exhibiting full recyclability in a facile, mild, and environmentally friendly way. This approach for assembling commercial small molecules into structurally complex materials paves the way for low-cost functional supramolecular materials based on synthetically simple procedures.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Self-assembly process of sodium thioctate in water. (A and B) Molecular structures (A) and schematic representation (B) of the ST monomers, ST polymers, and their networks. (C) Photographs of the ST crystalline powder, viscous ST polymer solution, and the resulting free-standing flexible solid film. (D) Schematic mechanism of the evaporation-induced interfacial supramolecular self-assembly from disordered polymers in aqueous solution to dry-ordered film network.
Figure 2
Figure 2
(A) Real-time detection of the formation of a crystalline-phase structure, upon water evaporation, by polarized optical microscopy. The colored bright spots indicated the presence of ordered crystallites in the corresponding region. (B) Photographs of a poly(ST) polymer film with natural light showing good transparency. (C) Photographs of a poly(ST) polymer film under polarized light suggesting the extensive presence of ordered crystalline regions. (D) SEM images of poly(ST) polymer film.
Figure 3
Figure 3
(A) UV–vis absorption spectra of ST aqueous solution (1 g/L) and dry poly(ST) film. (B) DSC curves of the ST powder and poly(ST) polymer film. (C) Hydration number (λ) measurements under varying relative humidity. (D) FT-IR spectra of the poly(ST) polymer film before (red line) and after (black line) release of adsorbed D2O.
Figure 4
Figure 4
(A) XRD patterns of the ST monomer powders and the resulting poly(ST) film. (B) Synchrotron radiation SAXS pattern of the poly(ST) film. Inset image shows the distinctive scattering ring of the sample. (C) One-dimensional GIWAXS plot of the poly(ST) film. Scattering intensity is plotted versus qz. (D) Two-dimensional GIWAXS pattern of the poly(ST) film.
Figure 5
Figure 5
(A) Tensile stress curves of the poly(ST) film under different RH (5%, 50%, and 80%, respectively). Inset photographs show the stretched poly(ST) film (RH = 80%). (B) Photographs show the rapid relaxation behavior of a stretched poly(ST) film (RH = 80%). (C) Schematic representation of the tension-induced alignment of the elastic poly(ST) film. (D) Optical microscope images of the polymer filaments made by stretching the hydrated poly(ST) films (RH = 80%). Bright field (top) and polarized light field (bottom) show the ordered fibers paralleled with the tension direction. (E) Schematic representation and optical images of the helical filaments which are formed by mechanical tension followed by relaxation.
Figure 6
Figure 6
(A) Schematic representation of the humidity-induced actuation behavior of poly(ST) polymer film. The blue arrows mean the humidity gradient direction. (B) Schematic representation of the expansion mechanism of the interlayers. (C) Photographs show the capability of poly(ST) polymer film acting as a humidity-responsive actuator. (D) XRD patterns of the poly(ST) film under varied RH. (E) Actuating kinetic curve of the bending polymer film to water vapor.
Figure 7
Figure 7
(A) Schematic representation of the water-mediated recycling process of the poly(ST) films. (B) Photographs show the recycling process of the polymer film fragments into a new polymer film. (C) Tensile stress curve of the original and recycled poly(ST) films. The tested samples are dried at room temperature (RH < 10%).
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